This invention relates to an apparatus for molding metals into articles of manufacture. More specifically, the present invention relates to an apparatus of the above type configured to increase thermal efficiency and increase through-put while decreasing thermal gradients and the resultant stresses.
Metal compositions having dendritic structures at ambient temperatures conventionally have been melted and then subjected to high pressure die casting procedures. These conventional die casting procedures are limited in that they suffer from porosity, melt loss, contamination, excessive scrap, high energy consumption, lengthy duty cycles, limited die life, and restricted die configurations. Furthermore, conventional processing promotes formation of a variety of microstructural defects, such as porosity, that require subsequent, secondary processing of the articles and also result in use of conservative engineering designs with respect to mechanical properties.
Processes are known for forming these metal compositions such that their microstructures, when in the semi-solid state, consist of rounded or spherical, degenerate dendritic particles surrounded by a continuous liquid phase. This is opposed to the classical equilibrium microstructure of dendrites surrounded by a continuous liquid phase. These new structures exhibit non-Newtonian viscosity, an inverse relationship between viscosity and rate of shear, and the materials themselves are known as thixotropic materials
While there are various specific techniques for forming thixotropic materials, one technique, an injection molding technique, delivers the alloy in an “as cast” state. With this technique, the feed material is fed into a reciprocating screw injection unit where it is externally heated and mechanically sheared by the action of a rotating screw. As the material is processed by the screw, it is moved forward within the barrel. The combination of partial melting and simultaneous shearing produces a slurry of the alloy containing discrete degenerate dendritic spherical particles, or in other words, a semisolid state of material and exhibiting thixotropic properties. The thixotropic slurry is delivered by the screw to an accumulation zone in the barrel which is located between the extruder nozzle and the screw tip. As the slurry is delivered into this accumulation zone, the screw is simultaneously withdrawn in a direction away from the unit's nozzle to control the amount of slurry corresponding to a shot and to limit the pressure build-up between the nozzle and the screw tip. The slurry is prevented from leaking or drooling from the nozzle tip by controlled solidification of a solid metal plug in the nozzle or by other sealing mechanisms. Once the appropriate amount of slurry for the production of the article has been accumulated in the accumulation zone, the screw is rapidly driven forward (developing sufficient pressure to force the solid metal plug, if necessary, out of the nozzle and into a receiver) thereby allowing the slurry to be injecting into the die cavity so as to form the desired solid article. Sealing the nozzle provides protection to the slurry from oxidation or the formation of oxide on the interior wall of the nozzle that would otherwise be carried into the finished, molded part. This sealing further seals the die cavity on the injection side facilitating the use of vacuum to evacuate the die cavity and further enhance the complexity and quality of parts so molded.
In the above technique, generally all of the heating of the material occurs in the barrel of the machine. Material enters at one section of the barrel while at a “cold” temperature and is then advanced through a series of heating zones where the temperature of the material is rapidly and, at least initially, progressively raised. The heating elements themselves are typically resistance or ceramic band heaters. As a result, a thermal gradient exists both through the thickness of the barrel as well as along the length of the barrel. As further discussed below, the thermal gradient through the barrel thickness is undesirable.
Typical barrel constructions of a molding machine for thixotropic materials have seen the barrels formed as long (up to 110 inches) and thick (outside diameters of up to 11 inches with 3 to 4 inch thick walls) monolithic cylinders. As the size and through-put capacities of these machines have increased, the length and thicknesses of the barrels have correspondingly increased. This has led to increased thermal gradients throughout the barrels and previously unforeseen and unanticipated consequences. Additionally, the primary material, wrought alloy 718 (having a limiting composition of: nickel (plus cobalt), 50.00–55.00%; chromium, 17.00–21.00%; iron, bal.; columbium (plus tantalum) 4.75–5.50%; molybdenum, 2.80–3.30%; titanium, 0.65–1.15%; aluminum, 0.20–0.80; cobalt, 1.00 max.; carbon, 0.08 max.; manganese, 0.35 max.; silicon, 0.35 max.; phosphorus, 0.015 max.; sulfur, 0.015 max.; boron, 0.006 max.; copper, 0.30 max.) used in constructing these barrels has previously been in short supply.
Since the nickel content of the alloy 718 is subject to be corroded by molten magnesium, currently the most commonly used thixotropic material, more recent barrel designs included a sleeve or liner of a magnesium resistant material to prevent the magnesium from attacking the alloy 718. Several such materials are Stellite 12 (nominally 30 Cr, 8.3W and 1.4C; Stoody-Doloro-Stellite Corp), PM 0.80 alloy (nominally 0.8C, 27.81 Cr, 4.11W and bal. Co. with 0.66N) and Nb-based alloys (such as Nb-30Ti-20W). Obviously, the coefficients of expansion of the barrel and the liner must be compatible to one another for proper working of the machine.
Reviews of failed barrels has yielded information that barrels fail often as a result of the thermal stress and more particularly thermal shock in the cold section or end of the barrels. As used herein, the cold section or end of a barrel is that section or end where the material first enters into the barrel. It is in this section that the most intense thermal gradients are seen, particularly in the intermediate temperature region of the cold section, which is located downstream of the feed throat.
During use of a thixotropic material molding machine as described above, the solid material feedstock, which has been seen in pellet and chip forms, is fed into the barrel while at ambient temperatures, approximately 75° F. Being long and thick, the barrels of these machines are, by their very nature, thermally inefficient for heating a material introduced therein. With the influx of “cold” feedstock, the adjacent region of the barrel is significantly cooled on its interior surface. The exterior surface of this region, however, is not substantially affected or cooled by the feedstock because the positioning of the heaters is directly thereabout. A significant thermal gradient, measured across the barrel's thickness, is resultingly induced in this region of the barrel. Likewise, a greater thermal gradient is also induced along the barrel's length. In this intermediate temperatures region of the barrel where the highest thermal gradients has been found to develop, the barrel is heated more intensely as the heaters cycle “off” less frequently.
Preheating of the barrel prior to production operation has also been long, up to three (3) hours. For example, a barrel having a 0.5 inch thick shrunk fit Stellite liner in a 1.85 inch thick alloy 718 shell, after normal preheating with ceramic band heaters for twenty minutes, the barrel will obtain an external temperature of about 700° F. (1200° F. is required for operation and molding of AZ91D magnesium alloy). At that same point in time, the thermal gradient through the barrel thickness is about 400° F. The barrel cannot be heated more intensely, and therefore faster, because of the generating of greater thermal gradients and stresses which can crack the barrel. Full preheating therefore requires about three (3) hours.
Prior metal processing machines have employed resistance type heaters. This heating technique generates the thermal energy within the resistance heater itself, which then must be transferred from the resistance heater to the barrel and other components of the machine. This means that the energy flow from the resistance heater to the part is maximized by a suitably large temperature differential. To accelerate this thermal transfer, one must obtain higher temperature differentials to overcome the thermal interface between the resistance heater (contact integrity) and the barrel, outer diameter through the barrel radial thickness, then into the feedstock and finally into the screw. Therefore, the energy level that is generated at the outside surface of the barrel, has to be high enough to sufficiently accelerate the energy flow to get uniform heating of the barrel, which therefore slows down the process and causes thermal fatigue of the barrel. Additionally, these resistance heaters, because of the thermal cycling they undergo, are also highly subject to thermal fatigue and frequent replacement. Another major problem is that the resistance heaters cannot couple thermal energy directly in the screw. As a result there are substantial thermal criteria in this arrangement which impact productivity and response to the thermal dynamics of handling incoming cold feedstock.
Within the barrel, a screw rotates, shearing the feedstock and moving it longitudinally through the various heating zones of the barrel. This causes the feedstock's temperature to rise and equilibriate at the desired level when it reaches the hot or shot end of the barrel. At the hot end of the barrel, the processed material exhibits temperatures generally in the range of 1050°–1100° F. The maximum temperature to which the barrel is subjected is near 1300° F. (for magnesium processing). As the feedstock is heated and moved through the barrel, the material is converted into a semisolid state where it develops its thixotropic properties.
Once a sufficient amount of material is accumulated in the hot section of the barrel and the material exhibits its thixotropic properties, the material is injected into a die cavity having a shape conforming to the shape of the desired article of manufacture. Additional feedstock is then introduced into the cold section of the barrel, lowering the temperature of the interior barrel surface, upon the ejection of the material from the barrel.
As the above discussion demonstrates, the interior surface of the barrel, particularly in the intermediate temperature region of the barrel, experiences a cycling of its temperature during operation of the injection metal molding machine. This thermal gradient between the interior and exterior surfaces of the barrel is dependent on barrel design, but has been seen to be as great as 227° F. during production operation.
Because of the significant cycling of the thermal gradient in the barrel, the barrel experiences thermal fatigue and shock. This has been found to cause cracking in the barrel and in the barrel liner in as little time as 30 hours. Once the barrel liner has become cracked, magnesium can penetrate the liner and attack the barrel. Both the cracking of the barrel and the attacking of the barrel by magnesium will contribute to the premature failure of the barrels. Molding machines can also operate in the all liquid state to inject good quality parts; but with the same needs for faster cycles and lower thermal stresses on the barrel as described above. As a variation, such machines can use a plunger rather than a screw for the injection stroke.
From the above it is evident that there exists a need for an improved construction, particularly one which decreases preheating times, decreases operation cycle times and decrease thermal gradients through the barrel thickness.
It is therefore a principle object of the present invention to fulfill that need by providing for an improved construction that optimizes heat transfer to and through-put of material being processed.
Another object of the present invention is to provide a construction decreasing preheating time
A further object of the present invention is to provide a construction that reduces thermal fatigue and shock in the barrel by reducing the thermal gradient through.